In direct thermal imaging the visible image pattern is formed by imagewise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
Such direct thermal imaging materials can be rendered photothermographic by incorporating a photosensitive agent which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density. Examples of photothermographic materials are the so called "Dry Silver" photographic materials of the 3M Company, which are reviewed by D. A. Morgan in "Handbook of Imaging Science", edited by A. R. Diamond, page 43, published by Marcel Dekker in 1991.
EP-A 692 391 discloses a heat-sensitive recording material being suited for use in direct thermal imaging and comprising in the order given:(i) a transparent polymeric support, (ii) a heat-sensitive imaging layer, and (iii) a protective layer characterized in that the protective layer is an opaque layer containing uniformly distributed in an organic hydrophilic polymeric binder at least one opacifying pigment in the form of particulate material some of which protrudes from the surface of said binder and has anti-stick properties with regard to a thermal print head, wherein the opacity of layer (iii) corresponds with an absorption and/or scattering of at least 80% of the light of the visible wavelength range (400 to 700 nm). The invention examples of EP-A 692 391 disclose a protective layer consisting of 11% by weight of gelatin and 89% by weight of opacifying anatase-type titanium dioxide particles.
W095/12495 discloses a method of recording an image by image- wise heating a recording material, the recording material comprising on the same side of a support, called the heat-sensitive side, (1) one or more layers comprising an imaging composition essentially consisting of (i) a substantially light-insensitive organic silver salt being in thermal working relationship with (ii) a reducing agent, and (2) at the same side covering the imaging composition a protective layer, characterized in that the image-wise heating proceeds with a thermal head contacting the heat-sensitive side and through the protective layer mainly comprising a cured polymer or cured polymer composition.
Ever tighter solvent emission regulations and measures to avoid solvent explosions, make the avoidance of solvent emission during the coating of thermosensitive elements with a protective layer desirable. However, coating with protective layers based on polyvinyl alcohol and tetra-alkyl orthosilcates, such as described in W095/12495, or polyvinyl alcohol and polysilicic acid as described in US 4,741,992, results in the emission of alcohols. There is therefore a need for water-based protective coatings for id thermosensitive and photo-addressable thermally developable elements based on substantially light-insensitive organic silver salts and reducing agents with good adhesion and transport properties in a thermographic printer using a thermal head, but without deterioration in thermographic imaging characteristics such as maximum and minimum print densities.